Vision disorders of the eye often relate to known primary defects in cone cells. These include macular dystrophies such as Stargardt's macular dystrophy, cone dystrophy, cone-rod dystrophy, Spinocerebellar ataxia type 7, and Bardet-Biedl syndrome-1, as well as color vision disorders, including achromotopsia, blue cone monochromacy, and protan, deutan, and tritan defects.
In addition to those disorders where the known cause is intrinsic to cone photoreceptors, there are vision disorders of the central macula (within primates) that may be treated by targeting cone cells. These include age-related macular degeneration, macular telangiectasia, retinitis pigmentosa, diabetic retinopathy, retinal vein occlusions, glaucoma, Sorsby's fundus dystrophy, adult vitelliform macular dystrophy, Best's disease, and X-linked retinoschisis.
A promising approach to treating and preventing ophthalmic disease that addresses the limitations of existing treatment is delivery of therapeutic agents to the eye with a gene therapy vector such as an adeno-associated virus (AAV). AAV is a 4.7 kb, single stranded DNA virus. Recombinant vectors based on AAV are associated with excellent clinical safety, since wild-type AAV is nonpathogenic and has no etiologic association with any known diseases. In addition, AAV offers the capability for highly efficient gene delivery and sustained transgene expression in numerous tissues, including eye, muscle, lung, and brain. Furthermore, AAV has shown promise in human clinical trials. One example is Leber's congenital amaurosis in which patients treated with a therapeutic delivered by a single subretinal administration of an rAAV vector have experienced sustained clinical benefit from expression of the therapeutic agent for more than four years from the initial date of treatment.
A number of challenges remain with regard to designing polynucleotide cassettes and expression vectors for use in gene therapy to treat eye disease generally and cone cells specifically. One significant challenge is obtaining sufficient expression of the transgene in target cells, especially in cone cells of the retina. A longstanding unmet need in the art has been sufficiently robust expression of transgenes following gene transfer. In some cases, more efficient expression is required for the efficacy of certain vectors, for example plasmid DNA vectors. In other cases, more efficient gene expression cassettes are desirable to allow for a lower therapeutic dose that has a more favorable safety profile or a less invasive route of administration (e.g., intravitreal vs. subretinal). In some settings, efficient expression has been achieved using a strong, ubiquitous promoter, but it is often desirable to have high transgene expression using a nucleic acid expression cassette that is only expressed in target cell types.
Previous efforts to express transgenes in cone cells, for example as disclosed in US patent application US 2012/0172419, showed some promise, but often the expression levels were lower than optimal or not cell specific. Given that a number of vision disorders result from primary defects in cone cells, specific expression of transgenes in cone cells, with high expression levels, would represent a meaningful advance in the art. Therefore, there remains a need for improved methods and optimized nucleic acid cassettes and vectors for expressing genes in cone cells.